1. Field of the Invention
The present invention is in the field of ultrasonic dispersion nozzles; more particularly, this invention is directed to an ultrasonic dispersion nozzle having a novel shut-off assembly and means for preventing the process fluid from interfering with the function of the mechanism.
2. Description of the Prior Art
Conventionally, spraying processes have used nozzles that rely on pressure and high-velocity fluid motion to atomize liquids. Generally, such nozzles are hydraulically operated devices in which pressurized liquid is forced through an orifice and sheared into droplets, or are of the two-fluid air-atomizing type in which high-pressure air or other gas mixes with liquid in the nozzle, and imparts a high velocity to the liquid, which is then ejected through the orifice. These nozzles are available in a wide variety of designs with numerous spray-shape patterns and flow-rate capacities.
However, such nozzles have various shortcomings which can cause operational and reliability problems. For example, although a high-velocity spray may be appropriate for some applications, it is undesirable in others because spray droplets can hit the surface to be coated with so high a velocity that some of them can bounce off. This overspray condition is not only wasteful, but can also result in the spray being dispersed into the atmosphere, thereby giving rise to environmental concerns.
Nozzle clogging is another persistent problem, especially when used with a material which has a tendency to solidify. Specifically, in order to achieve the high velocities required to break up the liquid, small-diameter channels and outlet orifices are needed. Because of these small diameters, however, the passageways are prone to blockage. This occurs when the fluid material dries in the orifices after the nozzle is shut off, when suspended particles gradually deposit in the nozzle, when foreign matter enters the fluid stream, or any combination of these and other factors. In order to remedy the first cause, the nozzle must be flushed after each use. In order to remedy the latter two causes, filtration is necessary. It will be appreciated that a completely blocked passageway results in total nozzle failure, while a partially blocked orifice or channel can cause a distorted spray pattern, or produce coarse droplets or decreased flow rates.
Further to the blockage problems, distortion in the spray pattern can occur when passageways are eroded by abrasive particles suspended in the liquid. Because of the high pressures and velocities used, even the hardest nozzle materials can be damaged within a relatively short time.
Various applications require the use of ultrasonic nozzles which avoid the aforementioned problems. Examples of such ultrasonic nozzles are those described in U.S. Pat. Nos. 4,153,201, 4,301,968, 4,337,896, and 4,352,459, all to Berger et al., assignors to Sono-Tek Corporation. With nozzles as described in those patents, atomization is achieved by vibrating a metallic surface at frequencies in the ultrasonic range; that is, above 20 kiloherz. Liquid is delivered to the atomizing surface through an axial feed tube running the length of the nozzle; for obtaining the necessary vibration, the nozzle incorporates piezoelectric transducers sandwiched between nozzle halves, whereby the vibrational motion is transmitted, amplified and concentrated at the atomizing surface.
As a result of such vibrations, a two-dimensional grid of capillary waves is formed in a liquid film on the atomizing surface of the nozzle. As the amplitude of the underlying vibration increases, the height of the surface wavelets also increases, until a critical amplitude is reached. At that time, the wave peaks become unstable, and are separated from the bulk liquid, whereby the material dispersed from the atomizing surface of the nozzle takes the form of drops smaller than or equal to the size of the wave crests on which they were formed. Since wavelength is inversely related to frequency, higher vibrational frequencies result in smaller droplets.
With such nozzles, since the atomization process is not pressurized, the diameter of the bore of the axial feed tube is unrestricted. Therefore, liquid emerges onto the atomizing surface at a low velocity, spreads out into a thin film, is atomized as described above, and is then directed toward the surface to be treated.
Ultrasonic nozzles provide distinct advantages over conventional nozzle arrangements. Specifically, the unpressurized operation results in a softer spray, with spray velocities being less than those typically produced by conventional nozzles by at least a factor of ten. Thus, spray material bouncing off the surface to be coated is substantially avoided, along with the aforementioned overspray condition. As a result, there is a resultant saving of expensive materials. Further, because unpressurized liquid is used, ultrasonic nozzles consume a minimum amount of power; for example, as little as four watts of electricity. Still further, because a large liquid-feed tube is used, for example, up to about 10 millimeters (mm), there is effectively a clog-free operation, even at supply rates of about 25 milliliters per hour (ml/hr). Other advantages include a large turn-down ratio, defined as the capability of producing droplets with median diameters as low as about 20 microns, and the ability to entrain the spray in a moving gas stream to define accurately a desired spray pattern and provide uniform coverage of large surface areas.
During intermittent processes, it is often important that there be a sharp cessation of fluid flow when the coating operation is terminated. In the two-fluid supply nozzles sold by Spraying Systems Co. described above, an internal shut-off assembly is provided which functions to interrupt only the liquid portion of the spray. Specifically, a stainless-steel shut-off needle is provided in the liquid-feed tube. An air-operated cylinder is provided to retract the shut-off needle against the force of a coil spring in order to start spraying. Because such nozzles operate under a high pressure and velocity, the shut-off needle does not effectively interfere with the supply of liquid. In such nozzles, since only the liquid-feed tube is closed, there is still an output from the high-velocity atomizing-air tube, unless separate provisions are made to terminate this stream.
It had previously been thought that an internal shut-off assembly could not be provided with an ultrasonic nozzle, because of predicted interference between the shut-off needle and the wave peaks which are formed. Instead, in order to discontinue liquid feed to an ultrasonic nozzle, particularly during intermittent operations, it is known in the art to install an automatic solenoid valve in the liquid-feed line upstream of the nozzle, and for the power supply for the piezoelectric transducers to be equipped with an interlock which attenuates the vibrations when the nozzle is off.
However, in actual tests with methanol and with an organotin-based coating formula containing monobutyltin trichloride, in which an interlock activated by a process timer was provided such that vibrations of the piezoelectric transducers were attenuated and with a two-way electric solenoid valve installed immediately upstream of the ultrasonic nozzle, it was found that liquid dripped from the orifice outlet of the nozzle upon discontinuation of the liquid feed. When the interlock was by-passed, liquid atomization continued from the nozzle tip for several seconds following discontinuation of the liquid feed. Such tests were performed with the ultrasonic nozzle mounted in a horizontal orientation and with a liquid-feed duration of approximately 0.5 second, such parameters being typical for commercial coating processes for fluorescent bulbs.
Failure to achieve a sharp cessation of liquid flow from the orifice of the nozzle in such applications is believed to be a result of the low surface tension of the liquids tested. As a class, liquid coating formulations to be applied to hot glass surfaces for the pyrolytic formation of a tin-oxide film thereon tend to have relatively low surface tensions.
In European Patent Application No. 81101985.0, published on Sept. 30, 1981, there is a suggestion that an internal shut-off assembly could be provided with an ultrasonic nozzle. Specifically, there is described a fuel-injection nozzle, the injection end of which is constructed as an ultrasound fluid atomizer having a working plate and truncated cone-shaped bending oscillator with piezoelectric motive power. The atomizer has a central bore with two different diameters defining a narrowing shoulder or seat; a nozzle pin or rod is slidably provided in the bore of the atomizer and has a frusto-conical end which seats on the shoulder so as to cut off the supply of liquid to the atomizing surface.
In a similar system, substantially described in U.S. Pat. No. 4,930,700, a 1.0-mm diameter tungsten shut-off needle is used, the free end of which is shaped to form a sealing tip near the spray orifice of the nozzle. A reduction in the bore diameter from about 1.7 to about 0.79 mm results in a shoulder against which the sealing tip of the needle engaged to form a metal-to-metal seal, and thereby cut off the fluid supply. The opposite end of the shut-off needle is coaxially inserted into a stainless-steel set screw, and silver-soldered therein. A nut of complementary size is silver-soldered to the set screw to simplify adjustment of the needle position. The shut-off needle assembly screws into a coaxial threaded hole in an actuator piston (or valve stem). The position of the shut-off needle is adjusted by varying the insertion depth of the set screw into the threaded hole in the actuator piston, and is fixed at such position by means of a lock nut. An O-ring or other seal means on the actuator piston provides a reciprocating seal between the piston and the inner walls of the shut-off assembly body, preventing the flow of process fluid into the actuator assembly.
However, such shut-off mechanism is not entirely suited for a plant environment. Initially, very slight leakage of coating chemicals past the reciprocating actuator-piston seal results in crystal growth on that sealing surface, thereby accelerating wear on the seal. Further, and related thereto, due to chemical attack by the coating chemicals, the various actuator components have a tendency to fail rather quickly. In addition, the silver solder of the shut-off needle to the set screw is wet by the coating process, and therefore subject to chemical attack.
Further, due to the large diameter increase from the shut-off needle or rod to the actuator-piston seal, from approximately a 1-mm diameter of the shut-off needle to the approximately 7.9-mm diameter of the actuator-piston seal, the internal volumne of the nozzle assembly changes substantially when the shut-off pin is opened or closed. This can result in a high-velocity slug of unatomized liquid exiting the nozzle while the shut-off pin is closing.
Still further, the shut-off mechanism-to-nozzle linkage is a mechanically weak point in the system. Because of such mechanical system, adjustment of the position of the shut-off pin requires disassembly of the mechanism. In addition, setting the correct position of the shut-off pin is a trial-and-error process and must be performed at a work bench, rather than at the plant site when in use.
Lastly, the choice of materials used to construct such a system is limited in view of the fact that many of the parts are wet by the coating chemicals. Thus, since the shut-off mechanism body is subjected to substantial mechanical loads, use of polymeric materials for corrosion resistance is not feasible.
Guthrie, in U.S. Pat. No. 4,536,140, discloses a positive-displacement piston pump for metering uniform pulses of a small amount of a coating chemical. In order to prevent piston seizure due to crystal formation resulting from minute leakage past the piston's reciprocating seals, a barrier fluid is provided between the piston wall and cylinder wall. However, the Guthrie patent is not directed to an ultrasonic nozzle.
Another process requirement is to direct and disperse the atomized liquid stream more accurately that was provided by the ultrasonic nozzle. Accurate direction is necessary to avoid overspray. Improved dispersion over the spray cone is necessary to avoid an extremely sharp boundary between coated and uncoated regions. This sharp boundary results in discoloration defects in coated fluorescent bulbs.